1. Field of the Invention
This invention relates generally to doctor blade holders of the type which employ self-compensating flexible support elements to load the doctor blades against the surfaces to be doctored, and is concerned in particular with an improvement in such support elements.
2. Description of the Prior Art
Doctor blade holders of the above-mentioned type are now well known to those skilled in the art. See for example U.S. Pat. No. 3,529,315 (Dunlap et al), the disclosure of which is incorporated herein by reference.
The self-compensating support elements employed in such blade holders usually comprise elongated thermoplastic flexible tubes filled with an incompressible liquid, e.g., oil. In order to perform satisfactorily, such tubes must necessarily have special cross-sectional configurations, with strategically arranged external ribs, and with wall portions having non-uniform thicknesses. For example, in a typical tube, the wall thickness will vary circumferentially from a minimum of about 0.16" at the top portion to a maximum of about 0.40" at the base portion.
Conventional end clamps employing metal inserts and externally swaged metal sleeves have been found to be ineffective in sealing the ends of such tubes. This is because the minimum thickness of the tube wall is often insufficient to withstand the pressure needed to form a proper seal between complimentary metal surfaces without rupturing. Also, the varying wall thickness of the tube causes the swaging force to vary correspondingly. Thus, the greater force required to establish a proper seal at the thinnest wall section often exceeds acceptable stress levels at the thicker wall section, thereby ultimately leading to ruptures at the thicker section.
These difficulties have caused those skilled in the art to abandon mechanical seals and to resort instead to heat sealing techniques. Here again, however, the results have been less than satisfactory. A major problem stems from the fact that during a heat sealing operation, the wall thickness near the seal is unavoidably reduced. If the thinnest wall section is overheated, its thickness will be reduced to the point where it can no longer resist rupturing under field conditions, which normally involve temperatures of up to 300.degree. F. and pulsing loads of up to 8 PLI. Overheating is extremely difficult to avoid because the entire cross section of the tube must be brought up to the melt point before a seal can be made. By the time the thicker wall sections are at that point, the thinner wall sections have often been overheated and excessively thinned.
A further problem with heat sealing stems from the fact that oil in the tube will tend to mix with and contaminate the molten tube material. Moreover, because the oil acts as a heat sink, even more heat must be applied to melt the tube, thereby exacerbating the difficulties associated with overheating.
Because of the foregoing problems, heat sealing operations have proven to be extremely time consuming, often taking a skilled operator in excess of one hour to seal both ends of one tube. In addition, heating sealing operations produce unacceptably high scrap losses, and with results that are largely inconsistent.
The heat sealed tube ends and the tapers associated therein are relatively long, usually about 2-1/2" in length, which considerably reduces the effective working length of the tube. Also, the heat sealed ends are highly susceptible to being damaged by flexing when being pushed during loading into the blade holder.